In large, high-speed electrical machines filled with hydrogen and in centrifugal compressors for dangerous gases, the shafts are sealed relative to the housing at the shaft housing passages with confining liquid, the sealing or confining liquid being kept in circulation. The liquid, preferably oil, fed to the seal under pressure produces in an annular gap a blocking action against an outflow of the gas from the machine. As a result of the solubility of the gas, the confining liquid absorbs air and passes the latter to the gas filling inside the machine, as a result of which the desired (or required) purity of the gas is (impermissibly) reduced. In order to prevent this, the confining liquid is therefore degassed under vacuum, which requires relatively large degassing appliances especially in the case of high gas pressures and/or large shaft diameters. Accordingly, in such plants, the gas losses and the tendency to foam are considerable. The gas losses are proportional to the volumetric flow of the confining liquid flowing off to the gas side and to the gas pressure.
In shafts having high peripheral speeds, i.e. having high speeds relative to the static sealing ring secured in the housing, in order to meet the demands of a satisfactory function in all operating states, the seals have to be made as so-called floating ring seals. Floating ring seals are understood, inter alia, as such liquid seals in which the liquid flowing off axially to both sides from the sealing location not only assumes the sealing function but at the same time also acts as a carrier for the frictional heat to be dissipated. This ensures, inter alia, that the sealing ring is not excessively deformed thermally, does not jam in the housing and does not graze on the shaft. It is known that accomplishing a sufficient heat dissipation by the liquid requires a sufficiently large radial operating clearance, as a result of which the (axial) leakage volumetric flow (in particular in the case of large shaft diameters) flowing in the direction of the machine interior also becomes very large and therefore the above operating requirement essentially cannot be fulfilled.
The leakage flow flowing off axially to the gas side from the sealing location has to be discharged from the machine again. However, a quantity of gas corresponding to the solubility also has to be constantly transported along with this leakage flow, which quantity of gas has to be removed from the liquid by suitable appliances (evacuating appliance) to prevent explosions or dangerous contamination of the atmosphere.
This quantity of gas which is removed entails, in particular in hydrogen-cooled turbo-generators, a flow loss which has to be replaced from an H.sub.2 reservoir and therefore is limited (cf. for example, DIN VDE 0530/Part 3).
Since hydrogen can escape not only at the shaft seal (controllable) but also at other uncontrollable sealing locations of the generator, the controllable loss at the shaft seal must be kept to a minimum.
Measures which keep this loss within permissible limits have hitherto remained restricted only to active elements (pumps) which circulate a liquid in a closed circuit in such a way that at the sealing location across a certain sealing gap the pressure gradient is on average zero per unit time. Such closed (secondary) liquid circuits which act as a gas-loss barrier, are used in special two-circuit shaft seals and in three-circuit shaft seals.
The closed secondary liquid circuit, apart from the circulation pump, requires for the satisfactory function of the seal the further components:
recooling appliance PA1 temperature control PA1 heating PA1 pressure regulating device PA1 filter for separating dirt particles PA1 minimum susceptibility to trouble at the shaft and sealing ring by avoiding the mixed friction (floating ring), PA1 minimum susceptibility to trouble in the oil supply system by 100% redundancy of the components, PA1 minimum surveillance, PA1 in particular in the case of the inner transfer (second alternative), no appreciable additional expenditure compared with single-circuit seals.
The pressure regulating device is required to ensure the same pressure between the secondary liquid circuit and the primary sealing liquid circuit.
For reasons of economy, the components of the secondary liquid circuit are constructed with no built-in redundancy. Therefore provision has to be made for the quantity of liquid which, in the event that the secondary circuit circulation pump fails, flows steadily from the primary circuit (main liquid circuit) toward the secondary circuit to be drawn off again, which requires an additional control line and an additional control regulator.
The most important disadvantage of the present solution is that, as a consequence of the fact that the pressure gradient in the sealing gap R is zero by design no convection cooling of the ring can take place in this area. The result is ring deformation (inversion) which is caused by thermal stress and leads to jamming of the ring in the housing when axial clearance is too small. Large axial clearance is undesirable on account of the oil leakage at the end face. However, jammed rings have lost the function of floating rings and over prolonged operating periods can cause damage to the material at ring and shaft as a result of unavoidable mixed friction effects.